U.S. Pat. No. 4,276,774 discloses apparatus for measuring the level of flowable material in which a cable is vertically suspended above and into the material, such that the material surrounds and supports the lower portion of the cable. The upper portion of the cable is periodically or selectively slackened against a weighing device, such as a load cell, that provides a signal indicative of weight of the slackened portion of the cable unsupported by material, and thereby indicates level of material with respect to the cable length. In one embodiment of the apparatus disclosed in the noted patent, the cable is suspended from a pneumatic actuator. In another embodiment, an electric motor is operable through a lead screw assembly for raising and lowering the cable, while in a third embodiment the cable is suspended from a pulley drive by an electric motor. U.S. Pat. No. 4,676,099 discloses suspended-cable apparatus of the subject type that embodies certain improvements having particular utility when employing steel laid cable.
Although apparatus of the above-described type, particularly as disclosed in the latter-noted patent, have enjoyed substantial commercial acceptance and success, improvements remain desirable in certain areas. For example, mechanisms in apparatus heretofore proposed for lowering and raising the cable have not proven to be entirely satisfactory for use in conjunction with long cables or chain-type cables that may be subjected to heavy loads as the cable is raised. Motor-driven cable suspension mechanisms proposed in the art are not sufficiently powerful and robust for most applications, while pneumatic actuators require provision of a compressed ar line, which is not always convenient in the field. Further, the suspended cable can be subjected to heavy shock loads when the surrounding vessel is being filled, and apparatus heretofore proposed have not provided a mechanism for absorbing such shock loads, and thereby preventing damage to the cable and suspension mechanism.
It is therefore an object of the present invention to provide suspended-cable apparatus of the described character that is particularly well adapted for operation in conjunction with long or heavy cables, and/or that includes facility for absorbing shock loads on the cable and thereby protecting the cable and suspension mechanism from damage. Another object of the present invention is to provide suspended-cable apparatus of the described character that satisfies industry standards for so-called explosion-proof housings, which are required for insurance reasons in many applications. Yet another object of the present invention is to provide suspended-cable apparatus of the described character that is economical to manufacture and install, and that is reliable in operation over an extended product life.
A further object of the present invention is to provide apparatus for measuring a preselected condition of materials, such as material level, over a predetermined operating range and for providing an analog current output over a selected range, such as the standard 4 ma to 20 ma current range, corresponding to the preselected material condition range, and in which the current output is closely regulated and compensated for variations that might otherwise occur due to temperature fluctuations, degradation of circuit operation, etc.